Just wondering, if we were to hypothetically accept all the side-fics as canon, approximately how large (square miles, kilometers, acres, hectares, etc. whatever) would Equestria have to be to incorporate all the locations mentioned? I'm assuming much larger than the continental US, at least.

The area around Ponyville, that is usually displayed in the show, is relatively small. Canterlot and Cloudsdale are likely less than 30 kilometers away from Ponyville. Old Appleloosa is probably at least a hundred kilometers away from Ponyville, probably more, considering it required an overnight rail trip to reach.

Based on earlier discussions, my own personal headcanon has Equestria (not counting zebra lands or anything else) as being roughly the size of Australia. That's about 7,692,024 square kilometers, which is not very different from the Continental US, at 7,663,941 square kilometers. I suspect that area is more than enough to comfortably host all of the side fics, especially since a lot of them probably overlap to a degree.